1. Field of the Invention
The present invention relates to a signal extraction method and apparatus in a wireless communication system. More particularly, the present invention relates to a signal extraction method and apparatus used in an interference canceling apparatus and the like which cancels degradation due to co-channel interference in communication channels.
2. Description of the Related Art
In the wireless communication system, technologies are known in which interference from an adjacent channel or interference from other systems are canceled by using antennas.
For example, “other route interference canceling apparatus” for canceling interference by using two Cassegrain antennas is known as one of the technologies. The other route interference canceling apparatus is configured as shown in FIG. 1.
The other route interference canceling apparatus includes send apparatuses 2 and 4 sending a desired signal (D) and an interference signal (I) respectively, antennas 1 and 3 of the send apparatuses, two Cassegrain antennas 5 and 6 directed to a desired wave and an interference wave, an interference canceling apparatus 7, a variable phase shifter 8, a variable attenuator 9, a subtracter 10 and a demodulator 11.
In FIG. 1, the Cassegrain antennas are installed such that one of the two Cassegrain antennas is directed to the interference wave and another is directed to the desired wave. In this example, the Cassegrain antenna 6 is directed to the interference wave direction and the Cassegrain antenna 5 is directed to the desired wave direction. Since the Cassegrain antenna has strong directivity to the front direction, a signal having relatively high S/N ratio (signal-to-noise ratio) can be obtained from each Cassegrain antenna. Thus, an output signal of the Cassegrain antenna 5 and an output signal of the Cassegrain antenna 6 are input to the subtracter 10 after adjusting phase and amplitude of the output signal of the Cassegrain antenna 6. At this time, the interference canceling apparatus 7 adjusts the phase and the amplitude of the output signal of the Cassegrain antenna 6 such that it generates opposite phase signal with respect to interference wave which is included in the output signal of the Cassegrain antenna 5. Therefore, interference wave component is removed from the output signal of the Cassegrain antenna 5. That is, according to this other route interference canceling apparatus, interference is removed by subtracting interference wave component from the output signal of the antenna directed to the desired wave by using the output signal of the antenna directed to the interference wave.
However, according to the above-mentioned technology, two Cassegrain antennas are necessary for removing interference. Therefore, the size of the apparatus becomes large.
In addition, “Side lobe canceler” is proposed from the viewpoint of suppressing increase of the apparatus size. The side lobe canceler is configured as shown in FIG. 2, for example.
As shown in FIG. 2, the side lobe canceler includes feed elements 24–27 which form array antennas, a beam forming network (BFN) 28, a subtracter 29, a square circuit 30, an antenna output terminal 31, multipliers 32–35, an adder 36, and an adaptive controller 37.
In this side lobe canceler, the beam forming network 28 is designed such that a main beam is directed to the desired wave direction and a sub beam can capture the interference wave which is input from a different direction. In this side lobe canceler, weight coefficient of the sub beam is determined and controlled adaptively by the adaptive controller 37 such that energy of sum of the main beam and the sub beam becomes minimum at the square circuit 30. Accordingly, interference wave incident from the side lobe of the main beam can be suppressed equivalently.
The other route interference canceling apparatus and the side lobe canceler are effective for a system such as a fixed wireless communication system or a geostationary satellite communication system in which interference wave direction and desired wave direction hardly move. However, the other route interference canceling apparatus and the side lobe canceler can not be applied to a system such as a mobile wireless communication in which the desired wave direction and the interference wave direction change speedily.
In the mobile wireless communication, “adaptive array” is used as a technology for removing interference. According to this technology, a desired output signal can be obtained by synthesizing input signals from each (feed) element of the array antenna by assigning weights optimally.
The adaptive array can be configured as shown in FIG. 3, for example. This adaptive array includes feed elements 13–16 forming the array antenna, multipliers 17–20, an adder 21, an antenna output signal terminal 22 and an adaptive controller 23.
In the adaptive array, input signals from the feed elements 13–16 are weighted by weight coefficient vector output from the adaptive controller 23, and are synthesized by the adder 21 so that desired wave component is extracted. That is, even when an interference wave source or a desired wave source is moved, desired wave can always be extracted by estimating the weight coefficients adaptively in the adaptive controller 23.
MMSE (Minimum Mean Squared Error) is known as an algorithm for estimating the weight coefficients. According to an adaptive array based on MMSE, directivity is adaptively controlled such that null of about 40 dB is formed in the direction of the interference wave in addition that the beam is directed to the desired wave. As a result, interference can be canceled strongly since the desired wave and the interference wave can be separated.
In addition, “space area multistage interference canceler” is known as a technology for removing interference in which the adaptive arrays are connected forming multistages. This space area multistage interference canceler is configured as shown in FIG. 4 for example.
As shown in FIG. 4, the space area multistage interference canceler includes an input terminal 163 which receives signals from all elements of the array antenna, subtracters 221–226, interference extraction units 201–209, adaptive array units 201–209, adders 178–181, adaptive array antennas 182–184, phase synchronization circuits 185–186, complex multipliers 187–188, discriminators 189–191, and an output terminal 192.
The interference extraction units 201–209 include adaptive array antennas 182–184, discriminators 189–191, phase synchronization circuits 185–186 and complex multipliers 187–188. The interference extraction unit also is a replica generator.
In the space area multistage interference canceler, the interference extraction units 201–203 and the subtracters 221–223 form a first stage, and the adders 178–179, the interference extraction units 204–206 and the subtracters 224–226 form a second stage. As a third stage and thereafter, the same configuration as the second stage is connected in tandem.
In a pth interference extraction unit of the first stage in the space area multistage interference canceler, an output signal from the adaptive array antenna is discriminated by the discriminator. Then, convolutional operation is performed on the discriminated signal and impulse response of the transmission line (output from the phase synchronization circuit 185, 186) by the complex multiplier so that a replica signal {circumflex over (r)}k,j,p(1) of the interference wave is generated, wherein r indicates the replica signal, (1) indicates the first stage, k indicates time, j indicates element number of antenna and p indicates the pth interference extraction unit.
The interference wave replica generated in this way is subtracted from the received signal from the array antenna element so that an error signal in which effect of the interference wave is eliminated is obtained at a point P. In the next second stage, interference extraction is performed in the same way as the first stage in which the error signal and the replica signal generated in the first stage are added in each corresponding interference extraction unit. Accordingly, as the stage proceeds, interference signal can be extracted under a condition that effect of interference is small. Thus, relatively good quality demodulation signal can be obtained.
However, as mentioned above, since the adaptive array antennas separate the desired wave and the interference wave by controlling directivity, interference can not be suppressed sufficiently when the desired wave and the interference wave enter the antenna from adjacent directions. In this case, the separation of the desired wave and the interference wave can be performed accurately by narrowing the beam of the array antenna by widening the interval of elements of the array antenna. However, when the interval of the elements is widened, interference wave increases so that diversity gain decreases.
In addition, when high-speed signal transmission is performed by using CDMA (Code Division Multiple Access), it can be estimated that delay waves of more than several number of symbols come from many directions so that interference waves increase. Thus, large number of elements are necessary to detect the interference waves for the above-mentioned adaptive array. As a result, the size of the apparatus and the amount of computation for removing the interference waves increase. Thus, power consumption can not be lowered.
On the other hand, although the space area multistage interference canceler serves for solving the above-mentioned problem of the adaptive array, the amount of computation increases in proportion to the product of the number of stages of the processing part which removes the interference wave and the number of interference waves which should be extracted. In addition, since the interference extraction unit includes complex circuits such as the adaptive array and the phase synchronization circuit, the apparatus cost increases. Further, as for the space area multistage interference canceler, like the adaptive array, when high-speed signal transmission is performed by using the access technology like CDMA, interference waves increase because delay spread and many multi-path waves intrinsic to the high-speed signal transmission occur. Thus, it is difficult to realize an apparatus which cancels thus increased interference waves by the space area multistage interference canceler.